1. Field of the Invention
The present invention relates to an improvement of a power conversion apparatus of such a type that DC power is inverted into AC power of a considerably high frequency, which is converted into a desired frequency after the voltage of the AC power has been adjusted at a desired value.
2. Description of the Related Art
There is known a power conversion apparatus, in which DC power is at first inverted by an inverter into AC power of a frequency sufficiently higher than the commercial frequency and, after the voltage of the AC power has been adjusted by a transformer at a desired value, the frequency thereof is converted by a frequency converter into the commercial frequency, whereby the transformer can be reduced in its size.
In Japanese patent laid-open publications Nos. JP-A No. 62/44072 and 62/44073 (both published on Feb. 26, 1987), for example, there is proposed a power conversion apparatus, in which there is used as a frequency converter a line-commutated converter consisting of two switching elements connected in reverse parallel with each other. As is well known, a line-commutated converter is a converter that the commutation of current from one of switching elements of the converter to another depends on the voltage of a power source or the voltage induced in a load, such as a counter electromotive force of a motor.
A power conversion apparatus having such a frequency converter has the following problem in the case where a phase difference exists between an output voltage of the apparatus and an output current thereof. Even though the phase of the output voltage is different from that of the output current, the power conversion apparatus can continue a normal operation, only if they are in the same polarity. However, switching elements of the apparatus can not commutate the current flowing therethrough, in the case where the voltage and the current are in the different polarity.
Then, it is considered to employ, instead of a line-commutated converter as described above, a self-commutated converter as a frequency converter, which is capable of carrying out the commutation of current from one of switching elements of the converter to another without depending on any external voltage, such as a source voltage or a load voltage. To this end, a self-commutated converter is necessary to be formed by switching elements, such as transistors, gate turn-off thyristors (GTOs) and so on, with the self turn-off function or those equivalent thereto.
Since, in such a self-commutated converter, there appears a so called spike voltage across switching elements of the converter due to a circuit inductance, such as a leakage inductance of a transformer, upon the commutation of the switching elements, there is usually provided a snubber circuit in parallel with each switching element in order to absorb the spike voltage to protect the switching element.
As is well known, a spike voltage is caused by the energy, which is stored in a circuit inductance and discharged therefrom, when a switching element is turned off. The spike voltage can be suppressed by making a capacitor of a snubber circuit absorb the energy discharged from the circuit inductance. Although the energy stored in the capacitor is further discharged therefrom, when the switching element turns on, it is only consumed by a resistor of the snubber circuit. Therefore, the energy stored in the capacitor of the snubber circuit becomes a loss, which is well known as a switching loss. For this reason, as the repetition frequency of the switching operation of a frequency converter becomes high, the switching loss occurring in snubber circuits increases, whereby the conversion efficiency of a power conversion apparatus becomes worse.